The multicatalytic proteinase complex (MPC, proteasome) is a ubiquitous cytoplasmic and nuclear particle which exists as a cylindrical complex assembled from 28 subunits. The MPC has multiple distinct protease activities and its active sites face a central water-filled canal "catalytic chamber" leading to degradation in an enclosed space with close proximity. This led to the suggestion that the lengths of the peptides formed during degradation depends on the distances between active sites in this "catalytic chamber". Experimental findings obtained with MPC from archeabacteria suggested that degradation of proteins by the MPC is processive. To test these mechanisms in MPC from a higher organism, we examined the size distributions of products formed early versus late in the course of protein degradation using reduced carboxyamidomethylated lysozyme (RCM-lysozyme) and MPC from bovine spleen and pituitary. The final degradation products ranged from 6 to 20 amino ac ids withou t a clear predilection for peptides of a particular size. This suggests that selection of cleavage sites is governed by the amino acid sequence specificity of the MPC catalytic sites and is not delimited by their spatial organization. Early in the course of degradation, 5 - 10 kDa peptides accumulated in > 80-fold molar excess over MPC. Mass spectrometry identified cont. several degradation products resulting from dual cleavages at both the N- and the C-terminus of RCM-lysozyme. Addition of the microviscogen sucrose slowed degradation of both denatured lysozyme and the large intermediates of degradation. The data indicate that degradation of proteins involves multiple, independent cleavages and dissociation of degradation intermediates. These findings indicate that degradation involved a non-processive mechanism in which initial cleavages lead into parallel pathways further degradation or dissociation. A general kinetic model for protein degradation by the MPC is discussed.